G protein-coupled receptors (GPCRs) are activated by a plethora of molecules including neuropeptides, polypeptide hormones and non-peptides such as biogenic amines, lipids, nucleotides and ions. They are classically composed of seven membrane-spanning domains and constitute one of the largest and most diverse gene families in the mammalian genome. Some novel GPCRs do not have obvious endogenous ligands and are termed orphan receptors, a number of which appear to be constitutively active. The cognate ligands for some of these orphan GPCRs have been identified, often based on the cellular and tissue distributions of the orphan GPCRs and occasionally using ‘reverse pharmacology’ where orphan GPCRs have been used to isolate novel endogenous substances. The human apelin receptor (APJ, gene symbol APLNR) first identified in 1993 is one such GPCR whose endogenous ligand, apelin, has been described.
Both APJ and apelin have been implicated as the key mediators of physiological responses to multiple homeostatic perturbations, including cardiovascular control, water balance, hypothalamic-pituitary-adrenal (HPA) axis regulation and metabolic homeostasis. Homeostatic stability is critical in mammalian organisms, and knowledge as to how this vital function is regulated and how this mechanism can go wrong in pathological conditions is still limited.
APJ was first identified as an orphan GPCR, with closest identity to the angiotensin II (Ang II) receptor, type AT1a. APJ remained an orphan receptor until 1998 when a 36-amino acid peptide termed apelin, for APJ endogenous ligand was identified. In the ensuing years, the receptor was deorphanised when its cognate ligand, apelin, was isolated from bovine stomach extracts. Recently, the apelinergic system has been shown to be critically involved in multiple homeostatic processes.
The protein structure of APJ is typical of a GPCR, containing seven hydrophobic transmembrane domains, with consensus sites for phosphorylation by protein kinase A (PKA), palmitoylation and glycosylation. The N-terminal glycosylation of GPCRs has been implicated in receptor expression, stability, correct folding of the nascent protein and ligand binding. Furthermore, the palmitoylation of the C-terminal tail has been reported to play a role in membrane association and, combined with receptor phosphorylation, these fatty acid modifications can influence the internalization, dimerization and ligand binding of a GPCR. Structural studies on APJ have determined that amino acids in both the N-terminal (e.g., Asp23 and Glu20) and C-terminal portions of the receptor are required for internalization.
The gene encoding human apelin, termed APLN, is located on chromosome Xq25-26.1 and possesses one intron within its open reading frame of ˜6 kb. In rat and mouse, the genes are termed Apln and are located at chromosomal locations Xq35 and XA3.2 respectively. The core promoter regions of these genes have been identified as −207/−1 and −100/+74 bp in rats and humans respectively. Similar to APJ, a CAAT box, but no TATA box, sequence is present in the rat and human promoter regions. Furthermore, rat and human preproapelin cDNAs do not have a classical Kozak consensus sequence surrounding the initiating methionine codon.
Human and bovine APLN cDNA sequences encode a 77-amino acid preproprotein (preproapelin) containing a hydrophobic rich N-terminal region, likely to be a secretory signal sequence. Bovine, human, rat and mouse preproapelin precursors have 76-95% homology and appear to exist endogenously as a dimeric protein, as a consequence of disulfide bridges formed between cysteine residues.
There are several mature forms of the apelin peptide. As the sequence of the purified peptide corresponded to the 36 C-terminal amino acids of the preproapelin protein, it was predicted that apelin-36 would constitute a mature form of the peptide. Additionally, as the C-terminal portion of preproapelin also contained lysine (Lys, K) and arginine (Arg, R) residues, and given their potential as sites for proteolytic cleavage, the existence of apelin-17 and apelin-13 peptides was predicted, along with a pyroglutamylated form of apelin-13 ((Pyr1)apelin-13). These mature forms of apelin lack cysteine residues and are probably only present in monomeric form. The likely secondary structures of apelin-36 and apelin-13 have been determined in aqueous solution, indicating that both possess an unordered structure. The amino acid sequence homology of the mature apelin-36 peptide is more conserved between species than that of preproapelin, with 86-100% homology between bovine, human, rat and mouse amino acid sequences, while the 23 C-terminal amino acids have 100% homology between species, suggesting an important physiological role.
Although APJ does not bind Ang II, apelin-13 shares a limited homology (four amino acids) with the vasoconstrictive peptide. Moreover, Ang I-converting enzyme 2 (ACE2), which catalyzes the C-terminal dipeptide cleavage of Ang I to Ang II, or Ang II to Ang 1-7, also acts on apelin-13 with a high catalytic efficiency, removing the C-terminal phenylalanine (Phe, F) residue. However, this cleavage may not inactivate the peptide, as the apelin isoform K16P, which lacks the terminal Phe, while ineffective at inducing receptor internalization or regulating blood pressure (BP) (effects associated with the full peptide), still binds to APJ and inhibits forskolin-stimulated cAMP production.
Although it is clear that APJ and apelin mRNAs and proteins are widely distributed in the CNS and peripheral tissues, whether the levels of mRNAs present in most of the regions of the brain and tissues are functionally relevant is not yet known.
Early studies of the expression of APJ mRNA by northern blot and quantitative PCR (qPCR) analyses have revealed strongest signals in the human caudate nucleus, corpus callosum, hippocampus, substantia nigra, subthalamic nucleus, medulla and spinal cord. Recently, the expression of APJ mRNA has also been demonstrated in the human cortex and hippocampus using a sensitive GPCR gene array profiling method—interestingly, APJ transcripts have also been detected in human bone marrow stromal cell lines. Transcriptomic analysis of multiple brain regions of human donors has revealed a widespread central expression of APJ mRNA with high levels in samples including the hippocampus (e.g., CA4 region), habenular nuclei, paraventricular nucleus (PVN) of the thalamus, supraoptic nucleus (SON) of the hypothalamus and various hindbrain structures. The salient feature of these studies is that APJ has been reported to have a widespread central distribution; although the function of APJ in the majority of brain regions is unknown, foremost among those regions probably important from a functional perspective include the PVN and SON of the hypothalamus.
In the periphery, the expression of human APJ mRNA was originally reported to be strongest in the spleen, with less expression being reported for the small intestine, colonic mucosa and ovary. A broader qPCR study has also reported strongest expression in the spleen, with high levels also being reported to be present in the placenta and weaker levels in the lung, stomach and intestine. (Pyr1)apelin-13-binding sites can be found within the media and intimal layers of muscular arteries and large elastic arteries and veins, while in the lung, apelin-binding sites have a predominantly vascular localization. Furthermore, APJ distribution in cardiovascular tissues, as demonstrated by immunohistochemistry (IHC), indicates APJ to be present in ventricular cardiomyocytes, vascular smooth muscle cells (VSMCs) and intramyocardial endothelial cells.
APJ binds numerous apelin isoforms and signals through various G proteins to a variety of signaling pathways to culminate in different patterns of activation and desensitization that may be tissue- and cell type-specific. Recently, APJ has also been reported to heterodimerize with other GPCRs and to signal in the absence of an endogenous ligand.
The C-terminal region of the apelin peptide may be responsible for its overall biological activity. N-terminal deletions of apelin-17 reveal that the 12 C-terminal amino acids may be the core requirements for the internalization and biological potency of APJ. Apelin-17 induces the internalization of APJ, which decreases with every N-terminal deletion to apelin-12, while the deletion of the terminal F amino acid results in a peptide that no longer internalizes APJ or affects arterial BP. The N-terminal residues within the RPRL motif (residues 2-5) of apelin-13 are critical for functional potency, and the C-terminal sequence KGPM (residues 8-11) is important for binding activity and for internalization. In contrast, the five N-terminal and two C-terminal amino acids of apelin-17 are not required for binding of the peptide to APJ or activation of receptor signaling (e.g., cAMP production). Although this may indicate a possible dissociation between the conformational states of the receptor responsible for receptor signaling and internalization, it is also possible that different ligand isoforms may induce differential receptor trafficking and signaling. These studies provide information on the structural importance of key apelin residues critical for efficient binding, activity and internalization, which have proved significant in the design and synthesis of apelin analogs.
Although progress has been made in recent years in clarifying the physiological significance of apelin/APJ, much remains to be discovered about the expression of the apelinergic system and precisely how it affects numerous physiological functions. Since the discovery of the apelin ligand, both apelin and APJ have been implicated as key regulators of central and peripheral responses to multiple homeostatic perturbations. These include playing pivotal roles in the regulation of cardiovascular function, angiogenesis, fluid homeostasis and energy metabolism and acting as neuroendocrine modulators of the HPA axis responses to stress. It is becoming apparent that the apelinergic system may play a pathophysiological role within many of these regulatory systems.
The central mRNA expression of preproapelin in regions of the hippocampus, hypothalamus, thalamus and midbrain shares a distribution pattern, as shown by ISHH, similar to that of angiotensinogen (Ang II precursor). Ang 1 is part of the rennin-angiotensin system (RAS), which controls extracellular fluid volume and arterial vasoconstriction, thereby regulating mean arterial blood pressure (MABP). The central actions of the RAS include the regulation of drinking behavior, salt appetite and VP secretion. Importantly, the RAS plays a critical role in the pathogenesis of heart failure. Interestingly, apelin exerts many physiological effects that appear to oppose those exerted by Ang II. More recently, apelin has been shown to block many Ang 1-initiated processes, perhaps partly by dimerization between APJ and AT1.
It is clear that apelin has both peripheral and central cardiovascular effects. However, experiments carried out in animal models have yielded conflicting results about the role of peripheral apelin in the regulation of vascular tone, with both pressor and depressor responses being described. In anaesthetized intact rats, the overall effect of peripherally administered apelin is the reduction of MABP. This hypotensive action is blocked by the NOS inhibitor L-NAME, indicating a nitric oxide-mediated pathway. In conscious rats, the effect is even less clear, with both increases and decreases in MABP being reported. Discrepancies among these reports may reflect the conscious state of the animal or the different apelin isoforms used in these studies; it is unknown which specific apelin peptide may be responsible for the (patho)-physiological roles of apelin. Further evidence that APJ plays a role in the regulation of BP comes from a study on mice with a global deletion of APJ, where a transient decrease in systolic BP observed in conscious wild-type (WT) mice following i.p. injection of (Pyr1)apelin-13 is abolished in APJ KO mice. However, while peripheral apelin is a vasodilator in the human saphenous vein, in vessels denuded of endothelium, apelin acts as a vasoconstrictor. Therefore, peripheral apelin may act as an antihypertensive factor, and sensitivity to the peripheral administration of apelin might be altered in hypertensive disease.
Additionally, the apelinergic system has an important role in cardiac function. In the isolated rat heart, infusion of apelin-16 induces a potent dose-dependent positive inotropic effect, with an EC50 of 40-125 pM in humans and ˜33 pM in rats, an effect also observed in the failing heart. In mice, administration of apelin increases myocardial contraction while reducing cardiac preload and afterload, without causing hypertrophy. Furthermore, apelin increases the shortening of sarcomeres in cardiomyocytes, an effect that is impaired in isolated ventricular myocytes from apelin and APJ KO mice. Apelin KO mice have an impaired response to cardiac pressure overload, thus suggesting a role for apelin/APJ in the sustainability and amplification of the cardiac response to stress. There is also evidence for a role in essential hypertension (EHT) as circulating levels of apelin-12 are decreased in patients with EHT. Functionally, the apelinergic system plays a role in the Cripto signaling pathway (which stimulates signaling by the transforming growth factor Nodal or growth/differentiation factors 1 and 3, via activin type IB and type IIB receptors) in mammalian cardiac myogenesis.
Cardiovascular development defects have been reported in APJ KO mice, where a loss of homozygous mutants has been described, but not in apelin KO mice, indicating possible ligand-independent effects of the receptor. This effect may perhaps be explained by the recent report that APJ signals independently of apelin in response to cardiac mechanical stretch. APJ KO embryos at E10.5, when lethality begins, have poorly developed vasculature of the yolk sac, delayed formation of the atrioventricular cushion and unusually formed cardinal veins and dorsal aorta. APJ KOs that survive do not reveal any apparent morphological differences; however, they have decreased vascular smooth muscle layer recruitment and myocardial defects including thinning of the myocardium, enlarged right ventricles and ventricular septal defects, suggesting an involvement of apelin/APJ signaling in cardiovascular development.
Apelin appears to have a role to play in the pathophysiology of the cardiovascular system—it has been implicated in vascular diseases, heart failure, and ischemia and subsequent reperfusion. In vascular diseases, the expression of apelin is up-regulated in the atherosclerosis of human coronary artery. Yet its role is undetermined, as conflicting evidence has been found in KO studies, indicating both antagonistic and inducing roles for apelin in atherosclerotic formation. During heart failure, plasma apelin levels rise in the early stages of disease and stabilize or lower as the condition develops. However, APJ mRNA is decreased in the weakened and enlarged heart of humans with idiopathic dilated cardiomyopath. Apelin may have a cardioprotective role in hypoxia and ischemia, where the cardiac levels of apelin and APJ respectively are increased. Apelin may also play a protective a role in ischemia/reperfusion injury, although the method of signaling appears to be independent of the characteristic myocardial kinase cascade, termed the reperfusion injury salvage kinase pathway. Post-infarct treatment with (Pyr1)apelin-13 reduces infarct size and increases HR, with a long-term antioxidant cardioprotective action.
Apelin is an angiogenic factor and mitogen of endothelial cells. Significantly, apelin is required for the normal development of frog heart and formation of murine blood vessels. Additionally, the development of the retinal vasculature is stunted in apelin KO mice, and apelin is necessary for hypoxia-induced retinal angiogenesis, and is also involved in non-neovascular remodeling of the retina.
The apelinergic system has been implicated in tumor neoangiogenesis. In brain tumors, the expression of apelin and APJ is up-regulated in microvascular proliferations, while tumor cell lines overexpressing apelin show increased growth. The pathophysiological effects of apelin in angiogenesis have also been reported for the liver, where the apelinergic system is a factor in portosystemic collaterization and splanchnic neovascularization in portal hypotensive rats as well as in neovascularization during liver cirrhosis. However, apelin may have therapeutic effects in ischemia recovery due to vessel regeneration and endothelial proliferation and blood vessel diameter regulation. These findings indicate that apelin is a crucial factor for angiogenesis and that there may be therapeutic potential in both the disruption of its signaling (e.g., tumors) and the stimulation of APJ expression (e.g., ischemia recovery).
The detection of APJ mRNA expression in areas of the brain critical for the control of fluid homeostasis led to the hypothesis that apelin may play a role in the regulation of body fluid balance. VP, along with OT, is synthesized primarily in the neurones of the mPVN and SON, which project to the posterior pituitary and release the peptides into the systemic circulation. The predominant endocrine function of VP from this source is to increase water permeability in the renal collecting duct cells, thereby allowing the retention of water.
The regulatory actions of apelin on thirst and drinking behavior have been reported. In water-replete animals, a significant increase in water intake is observed following i.p. or i.c.v. injection of apelin, whereas in other studies apelin has been reported to reduce water intake post i.c.v. injection or to have no effect. Additionally, in water-deprived rats, an inhibitory effect or lack of any effect of apelin on drinking behavior is observed, while in apelin KO mice, the dehydration-induced drinking response is comparable to that observed in WT mice. The expression of apelin and APJ mRNAs, and labelling of apelin-immunoreactive magnocellular cells, are increased by dehydration, while the labelling of VP-immunoreactive cells decreases, implying the differential regulation of these peptides in response to dehydration. Recently, however, abnormal fluid homeostasis has been demonstrated in APJ KO mice, manifested by a decrease in drinking behavior and an inability to concentrate urine to levels observed in controls during water deprivation, suggesting an antidiuretic effect of apelin in vivo. However, in lactating rats, apelin induces diuresis and has direct effects on renal vasculature. APJ is also necessary in dehydration-induced signaling in the subfornical organ, implicating the apelinergic pathway in responses to hyperosmotic stimuli.
A number of studies have pointed out an emerging involvement of apelin in energy metabolism and a role for adipocyte-derived apelin in the (patho)-physiology of obesity has been reported. Both apelin and APJ mRNAs are present in mouse, human and rat adipose tissue, and their levels increase in adipose tissue and plasma with obesity. This highlights APJ as an intriguing therapeutic target for metabolic disorders. However, the expression of plasma apelin is increased only in obese humans and in mouse models of obesity associated with hyperinsulinemia, indicating that obesity or high-fat feeding may not be the main cause for the rise in the expression of apelin, and implying a close relationship between apelin and insulin both in vivo and in vitro. Insulin directly acts on adipocytes in vitro to stimulate the production of apelin, and the expression of apelin mRNA is down-regulated in the adipocytes of mice treated with the β-cell toxin streptozotocin, which leads to a fall in plasma insulin levels. In mice, nutritional status influences apelin levels in vivo—fasting inhibits plasma levels, which are then restored by re-feeding—thus strengthening the implication that insulin regulates apelin gene expression and secretion. Additionally, apelin, perhaps through APJ expressed in pancreatic islet β-cells, regulates the secretion of insulin—apelin inhibits glucose-stimulated insulin secretion in vivo in mice and in isolated islets of Langerhans in vitro. Interestingly, in a recent study, apelin has been shown to alleviate diabetes-induced reduction of pancreatic islet mass and to improve the insulin content of pancreatic islets in type 1 diabetic mice.
Apelin may have a positive effect in the metabolic syndrome (a combination of risk factors that when occurring together increase the risk of coronary artery disease, stroke and type 2 diabetes (T2D)). Apelin KO mice have reduced insulin sensitivity, are glucose intolerant and are hyperinsulinemic. The peripheral administration of apelin reduces peak plasma glucose concentrations by increasing glucose uptake in skeletal muscle and adipose tissue and improves insulin sensitivity in both apelin KO and obese high-fat diet fed mice, with the insulin-sensitizing effects continuing for up to 4 weeks, with no tolerance to the actions of apelin. Apelin increases glucose uptake, both in vitro and in vivo, through both insulin-dependent and -independent pathways. Apelin may also decrease body adiposity, independently of altered food intake, by increasing energy expenditure through the activation of mitochondrial uncoupling proteins 1 and 3. Clinical studies have shown a promising therapeutic value for apelin, as apelin displays beneficial glucose-lowering effects in human adipose tissue and plasma apelin levels correlate with glucose and HbA1c levels. Apelin is linked to the pathogenesis of T2D—plasma apelin concentrations are increased in insulin-resistant patients, in type T2D patients and in morbidly obese T2D individuals, perhaps indicating a compensatory role of apelin in the reduction of insulin resistance. However, conversely, plasma apelin levels are reduced in newly diagnosed T2D patients and increased in T2D patients and obese non-diabetic individuals. The increased expression of apelin in plasma and adipose tissue of obese individuals can, however, be reversed by a hypocaloric diet. As a result of such studies, similarities between the function of apelin and that of insulin, and a link between this adipokine and glucose homeostasis, have been hypothesized.
As has been noted previously, APJ is localized in the hypothalamic pPVN and the anterior pituitary gland, key areas involved in the stress response. Apelin mRNA is also present in these areas, co-localizing with VP in the mPVN, SON and pituitary. Additionally, apelin immunostaining of cell bodies and fibers is highest in the hypothalamus, with large numbers of apelin-positive cell bodies present in the PVN and SON. The presence of APJ and apelin in VP- and CRH-containing hypothalamic nuclei, which are pivotal to the HPA axis responses to stress, suggests a role for apelin/APJ in neuroadenohypophysial hormone release.
A role for apelin in the regulation of the HPA axis responses to stress is supported by studies showing that central administration of (Pyr1)apelin-13 increases the expression of c-fos, an indicator of neuronal activity, in the PVN. Furthermore, administration of apelin-13 stimulates the release of CRH and VP from hypothalamic extracts in vitro, effects consistent with stimulation of the stress axis. APJ mRNA levels increase in the PVN in response to acute and chronic stress and following adrenalectomy, implying negative regulation of the expression of APJ mRNA by glucocorticoids. Additionally, dexamethasone, a glucocorticoid agonist, decreases apelin mRNA levels in 3T3-L1 mouse adipocytes.
Apelin may potentially stimulate the secretion of ACTH either directly at the level of the pituitary corticotroph or via an indirect action on the hypothalamus involving the release of both VP and CRH. Consistent with the expression of apelin and APJ in anterior pituitary corticotrophs, administration of apelin-17 directly increases the release of ACTH, while also augmenting K+-stimulated ACTH release, in an ex vivo perfusion system of anterior pituitary glands, suggesting possible autocrine or paracrine functions for apelin in this tissue. Central administration of (Pyr1)apelin-13 in rats also increases plasma ACTH and CORT levels while decreasing prolactin, luteinizing hormone and follicle-stimulating hormone levels. However, increases in plasma ACTH and CORT levels observed after i.c.v. administration of (Pyr1)apelin-13 in mice are reduced to control levels by pre-treatment with the CRH receptor antagonist α-helical CRH9-41, while (Pyr1)apelin-13-mediated increases in plasma ACTH levels are abolished in VP V1b receptor KO mice, indicating that apelin also modulates the release of ACTH via an indirect action on the hypothalamus involving both CRH- and VP-dependent mechanisms. Recently, using APJ KO mice, APJ has been shown to play a regulatory role in the modulation of the HPA axis responses to some acute stressors including LPS challenge (an immune stressor), insulin-induced hypoglycemia (a metabolic stressor) and forced swim (a physical/psychological stressor). These studies suggest that other peptides cannot compensate for the loss of APJ to directly, or indirectly, induce the release of ACTH in response to stress. Thus, the integration of neurobehavioral responses to stress may be more complicated than previously envisioned, with apelin/APJ exerting a pivotal neuroregulatory role.
Apelin was first isolated from stomach extracts, and studies on the actions of apelin in the gastrointestinal system have found functional, and possible cell survival, roles. In the gastrointestinal system, apelin/APJ may be regulators of hormone and gastric acid secretion. Apelin/APJ may also have a direct effect on vascular smooth muscle, including vasoconstriction, which may affect renal glomerular hemodynamic function in the rat kidney. Some studies have also proposed an immunological role for apelin as it reduces the production of cytokines in mouse spleen cells, suggesting that apelin may modulate neonatal immune responses through rodent and bovine colostrum and milk. APJ is also a co-receptor of HIV entry into target cells, an action that is blocked by apelin. APJ may contribute to HIV-1 infection and pathogenesis in CNS-based cells as viral envelope proteins can mediate fusion with APJ-positive, cluster of differentiation 4 (CD4)-negative cells, provided that CD4 is added in trans, and HIV can infect APJ-expressing cells despite their CD4 status. Other possible roles for apelin and APJ in the rodent CNS include antinociception, enhancement of depressive behavior, and facilitation of passive avoidance learning. Apelin may also have a role in neuroprotection, as apelin pre-treatment protects hippocampal neurones against N-methyl-D-aspartate (NMDA) receptor-mediated excitotoxic injury, possibly via the phosphorylation of Akt and ERK1/2, and prevents apoptosis in cultured mouse cortical neurones.
Furthermore, apelin and APJ are expressed in osteoblasts where they may induce cell proliferation and promote survival; however, an increase in bone mass can be observed in apelin KO mice. Recently, apelin has been reported to have a potential role in the pathophysiology of osteoarthritis (OA), as apelin is present in synovial fluid, and OA patients have elevated plasma apelin concentrations. Blood plasma levels of apelin are reduced in patients with polycystic ovary syndrome, consistent with the role played by apelin/APJ in metabolic disturbances such as insulin resistance.
Elevated levels of apelin have been detected in many pathological states or disease processes, such as heart disease, atherosclerosis, tumor angiogenesis and diabetes. However, in many systems, apelin has been shown to have positive effects, for example in the cardiovascular system, where it has a cardioprotective effect. This has led to speculation that apelin and APJ could be beneficial targets for therapeutic strategies for a number of diseases and disorders.
To date, there are few reports of selective small molecule apelin receptor agonists, and thus far none of the reported agonists have favorable pharmacokinetic profiles. There is a need, therefore, for potent compounds that target and exhibit dose dependent agonism of the apelin receptor.